Roast or steam the sweet potatoes. Preheat oven to 400°F (200°C).
Toss cubes with a drizzle of olive oil, salt, and pepper. Roast on a sheet pan for 18–22 minutes until tender and lightly browned. Alternatively, steam until fork-tender.
Sauté aromatics. In a large skillet, warm 1 tablespoon olive oil over medium heat.
Add onion and bell pepper; cook 5–6 minutes until softened. Stir in garlic for 30 seconds until fragrant.
Season and combine. Add cumin, chili powder, smoked paprika, oregano, cinnamon (if using), salt, and pepper. Stir for 30 seconds to bloom.
Fold in black beans and roasted sweet potatoes. Add lime juice and cilantro. Taste and adjust salt and spice.
Make the enchilada sauce. In a saucepan, heat 2 tablespoons olive oil over medium.
Whisk in flour and cook 1 minute. Add chili powder, cumin, garlic powder, onion powder, and oregano; whisk 15 seconds. Slowly whisk in broth.
Simmer 5–7 minutes until slightly thickened. Stir in vinegar, salt, and a pinch of sugar if needed.
Warm the tortillas. To prevent tearing, wrap tortillas in a damp paper towel and microwave for 30–45 seconds, or briefly heat in a dry skillet.
Assemble. Spread 1/2 cup sauce on the bottom of a 9x13-inch baking dish. Add 2–3 tablespoons of filling to each tortilla, sprinkle a little cheese if using, then roll and place seam-side down in the dish.
Top and bake. Pour remaining sauce over the enchiladas, spreading evenly.
Add remaining cheese if using. Bake at 375°F (190°C) for 18–22 minutes, until bubbly and warmed through.
Finish and serve. Let rest 5 minutes. Top with avocado, cilantro, and a squeeze of lime.
Serve hot.